[Gentoo] Configuring what hardware to build X for

I’ve added VIDEO_CARDS capabilities to x11-drm and mesa in addition to xorg-x11 now, so you have complete control over which video cards and input devices your X will support. If you happen to use binary drivers, you’ll probably want to set VIDEO_CARDS=”none fglrx” or “none nvidia”; that will prevent the 3D drivers in mesa from building.

Makes for a nice reduction in space — each 3D driver on my system takes up about 13M, when built with debugging. Installing 7 instead of 15 makes quite a difference. Also there’s the savings in build time.

If people clamor enough, I might split ati out into r128, radeon and mach64, as well, but probably won’t split radeon up any further.